The present invention relates to large cooling systems, such as for buildings and large equipment, and more particularly relates to an apparatus providing proportioned softened makeup water for a cooling system where concentration of impurities in recirculating water can cause system fouling. The present invention also relates to fail-safe methods of controlling pretreatment processes, including use of a conductivity sensor that monitors low conductivity as a function of system control. However, the present innovation is not limited to only the above noted applications, and instead it is contemplated that it will work for many water and fluid treatment systems, including ones that use demineralized water, reverse osmosis product water, nano-filtrated water, and other pretreatment processes (e.g. precipitation processes to remove silica from water).
Many cooling systems are water-cooled by means of recirculated water that partially evaporates, such as will occur when passing over a cooling tower during the process. The evaporated water leaves mineral free, but minerals and other impurities in the water are left behind in the recirculated cooling water. The evaporated water is replaced from the cooling system makeup source, which typically introduces more minerals into the system. Unless separate action is taken, the mineral concentration in the cooling water will continue to increase, until the solubility limit of the minerals is exceeded. At this point, precipitation occurs, and scaling or other fouling of the system may result. To prevent this, cooling water is intentionally removed from the system. This elimination is referred to as blowdown (or sometimes “bleed-off”).
The degree of concentration of the cooling water is typically expressed as a multiple of the concentration of the makeup source, and is called the “cycles of concentration”. There is a mathematical relationship between the cycles of concentration (“c”), evaporation (“E”), blowdown (“B”) and makeup (“M”). This relationship is described by the following two equations:M=E+B  (1)B=E/(c−1)  (2)
As seen in the above equations, as the value of c increases, the value of blowdown B is reduced. Also, as the blowdown B is reduced, the total makeup requirement for the cooling system is reduced.
By way of example: Consider the use of a 1000 ton chiller with a cooling tower. Assuming that the system is utilized at full capacity, about 30 gallons per minute (gpm) of evaporation will nominally be required to provide the designed heat rejection. If this system is operated at three cycles of concentration, then an additional 15 gpm (B=E/(c−1) or 50% of the evaporation, will be required to maintain this concentration. The total makeup would therefore be 45 gpm (30 gpm+15 gpm). However, as shown in the chart below, the blowdown (and makeup) requirement changes with the increase in cycles of concentration:
Cycles34567E3030303030B15107.565M454037.53635
In the above example, the ability to increase the allowable cycles of concentration from three to six cycles yields a savings of 9 gpm, or 12,960 gallons/day (gpd), or 4,730,400 gallons per year. With the rising costs of water and sewerage charges, this water savings can save over $30,000 annually.
The upper limit of cycles of concentration is often limited by the makeup source quality and the saturation limits of its constituent minerals. For example, water hardness (especially calcium compounds) must often be limited in the recirculated water to prevent precipitation (and concomitant scaling). Using the example above, let us assume that the cycles of concentration in an evaporative cooling system were limited by the saturation limit of hardness in the cooling water, and that the concentration of total hardness is limited to 500 ppm. In this example, cycles of concentration would necessarily be limited to 500/150 or 3.33 cycles. And, a priori, the required blowdown rate would be 0.43 (i.e. 1/(3.33−1)) of evaporation.
One known water saving method is to partially purify the makeup water, effectively reducing the mineral loading in the cooling water and allowing a greater cycles of concentration to be achieved at the same given saturation limit.
A water softener removes the hardness from water, usually by means of ion exchange. In our example, the makeup water (containing 150 ppm of total hardness) would pass through the softener, and the effluent from the softener would nominally have less than 1 ppm of total hardness. By partially softening the makeup source (i.e. softening removes the hardness) using an ion exchange resin (i.e. water softener), the system yields an overall reduction in the amount of calcium introduced to the cooling water by the makeup. This lower amount would allow for greater cycles of concentration, and a lower total makeup.
Typical evaporative cooling systems include a water level control employing a float valve or an electronic level sensor and a solenoid valve, with the float valve/sensor/solenoid-valve device(s) being located outdoors at the cooling tower itself. Known systems that locate these devices outdoors do this despite the fact that the installation of such devices to partially purify (i.e. soften) the water in an outdoor environment is more complex and expensive than an indoor installation. If a water softener device is used in the above “typical” system is located inside, then the “typical” system (using traditional thinking) would require a substantial amount of piping and wiring (and expense) to tie into the (distant) makeup piping and (remote) cooling tower, which includes routing concerns, and safety and construction concerns/requirements.
Apparatus and methods are desired for cooling systems to simplify installations and constructions, reduce installation costs and construction costs, and improve an ability to maintain, service, and check on water softening equipment used, as well as optimizing the system to maximize cycles of concentration of system water prior to the need to dump mineral-laden system water and replace it (and concurrently, minimize the amount of water sent to drain).